1. Field of the Invention
This invention pertains generally to the precision measurement of electromagnetic fields and more specifically to a two-dimensional imager for measuring the electric-field amplitude and phase distribution across an extended spatial region.
2. Description of the Related Art
Radiating antenna apertures require a precise measurement of electro-magnetic fields which are coupled into free space. Precise knowledge of the electromagnetic field distribution in either near or far-field is required for a complete system performance characterization. The measurement provide information on the main beam shape and pointing angle, on sidelobe levels and null depths, on the presence of the deleterious grating lobes, and on element mutual coupling effects.
In the prior art, electronic near-field measurements utilize a field probe mechanically scanned above the radiating aperture. See, Rahmat-Samii et al., The UCLA Bi-polar Planar-Near-Field Antenna-Measurement and Diagnostics Range, IEEE Ant. and Prop. Mag., Vol. 37, No. 6, pp. 16-35, Dec. 1995. The constraints of such a system are imposed by the mechanical movement requirements such as: shielding of the support structure to minimize invasiveness and multiple reflections, mechanical probe position errors that may significantly corrupt the calculated far-field patterns, and phase errors due to the flexing microwave signal cables. Furthermore, such near-field measurement systems require precise probe calibration, are only suitable for a laboratory environment, and may not be usable on high-power systems.
Electronic far-field radar range measurements have many of the same limitations as the near-field measurements. The mechanical positioning and probe calibration errors are mitigated since the measurement is done in the far-field. However, this comes at the expense of significant real-estate demands to satisfy the far-field requirement. Additional errors are introduced by the large separation of the transmitter and the receiver measurement points requiring long distance transmission of high-frequency reference signals. The technique is only suitable for a laboratory environment and is not usable on high-power systems.
Compact range measurements suffer from requirements of the mechanical antenna scans, cannot be used for high-power systems, and require large specialized anechoic chambers.
Optical near-field measurements with probes require precise mechanical movement to mitigate positioning errors, require frequent electric (E)-field calibration, shielding of the surrounding system, and are suitable only for laboratory environments. See, Imaizumi et al., Electric Field Distribution Measurement of Microstrip Antennas and Arrays Using Electro-Optic Sampling, IEEE Trans. on Micro. and Techns., Vol. 43, No. 9, pp. 2402-2407, Sep. 1995.